zpoolprops.8: clarify vdev expansion rules

Remove reference to EFI(?), explain that the new space
is beyond the GPT for whole-disk vdevs, and add section noting how it
behaves with partition vdevs in terms of how the user is most likely to
encounter it ‒ the previous phrasing was confusing
and seemed to indicate that "zpool online -e" will be able to claim

  GPT[whatever, ZFS, free space, whatever]

into

GPT[whatever, ZFS, whatever]

but that's not the case, as it'll only be able to do so after manually
resizing the ZFS partition to include the free space beforehand, i.e.:

  GPT[whatever, ZFS, free space, whatever]
  GPT[whatever, [ZFS + free space], potentially left-overs, whatever]
  # zpool online -e
  GPT[whatever, ZFS, whatever]

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11158